Abstract. A new ice core was drilled in West Antarctica on Skytrain Ice Rise in field season 2018 / 2019. This 651 m ice core is one of the main targets of the WACSWAIN (WArm Climate Stability of the West Antarctic ice sheet in the last INterglacial) project. A present-day accumulation rate of 13.5 cm w.e./year was derived. Although the project mainly aims to investigate the last interglacial (115–130 ka BP), a robust chronology period covering the recent past is needed to constrain the age models for the deepest ice. Additionally, this time period is important for understanding current climatic changes in theWest Antarctic region. Here, we present a stratigraphic chronology for the top 184.14 m of the Skytrain ice core covering the last 2000 years based on absolute age tie points interpolated using annual layer counting encompassing the last 2000 years of climate history. Together with a model-based depth-age relationship of the deeper part of the ice core, this will form the ST22 chronology. The chemical composition, dust content, liquid conductivity, water isotope concentration and methane content of the whole core was analysed via continuous flow analysis (CFA) at the British Antarctic Survey. Annual layer counting was performed by manual counting of seasonal variations in mainly the sodium and calcium records. This counted chronology was informed and anchored by absolute age tie points, namely, the tritium peak (1965 CE) and six volcanic eruptions. Methane concentration variations were used to further constrain the counting error. A minimal error of ± 1 year at the tie points was derived, accumulating to ± 5–10 % of the age in the unconstrained sections between tie points. This level of accuracy enables data interpretation on at least decadal timescales and provides a solid base for the dating of deeper ice, which is the second part of the chronology.
Abstract. Climate during the last glacial period was marked by abrupt instability on millennial timescales that included large swings of temperature in and around Greenland (Daansgard–Oeschger events) and smaller, more gradual changes in Antarctica (AIM events). Less is known about the existence and nature of similar variability during older glacial periods, especially during the early Pleistocene when glacial cycles were dominantly occurring at 41 kyr intervals compared to the much longer and deeper glaciations of the more recent period. Here, we report a continuous millennially resolved record of stable isotopes of planktic and benthic foraminifera at IODP Site U1385 (the “Shackleton Site”) from the southwestern Iberian margin for the last 1.5 million years, which includes the Middle Pleistocene Transition (MPT). Our results demonstrate that millennial climate variability (MCV) was a persistent feature of glacial climate, both before and after the MPT. Prior to 1.2 Ma in the early Pleistocene, the amplitude of MCV was modulated by the 41 kyr obliquity cycle and increased when axial tilt dropped below 23.5∘ and benthic δ18O exceeded ∼3.8 ‰ (corrected to Uvigerina), indicating a threshold response to orbital forcing. Afterwards, MCV became focused mainly on the transitions into and out of glacial states (i.e. inceptions and terminations) and during times of intermediate ice volume. After 1.2 Ma, obliquity continued to play a role in modulating the amplitude of MCV, especially during times of glacial inceptions, which are always associated with declining obliquity. A non-linear role for obliquity is also indicated by the appearance of multiples (82, 123 kyr) and combination tones (28 kyr) of the 41 kyr cycle. Near the end of the MPT (∼0.65 Ma), obliquity modulation of MCV amplitude wanes as quasi-periodic 100 kyr and precession power increase, coinciding with the growth of oversized ice sheets on North America and the appearance of Heinrich layers in North Atlantic sediments. Whereas the planktic δ18O of Site U1385 shows a strong resemblance to Greenland temperature and atmospheric methane (i.e. Northern Hemisphere climate), millennial changes in benthic δ18O closely follow the temperature history of Antarctica for the past 800 kyr. The phasing of millennial planktic and benthic δ18O variation is similar to that observed for MIS 3 throughout much of the record, which has been suggested to mimic the signature of the bipolar seesaw – i.e. an interhemispheric asymmetry between the timing of cooling in Antarctica and warming in Greenland. The Iberian margin isotopic record suggests that bipolar asymmetry was a robust feature of interhemispheric glacial climate variations for at least the past 1.5 Ma despite changing glacial boundary conditions. A strong correlation exists between millennial increases in planktic δ18O (cooling) and decreases in benthic δ13C, indicating that millennial variations in North Atlantic surface temperature are mirrored by changes in deep-water circulation and remineralization of carbon in the abyssal ocean. We find strong evidence that climate variability on millennial and orbital scales is coupled across different timescales and interacts in both directions, which may be important for linking internal climate dynamics and external astronomical forcing.
Abstract. Atmospheric nitrogen oxides (NO and NO2) were observed at Dome C, East Antarctica (75.1° S, 123.3° E, 3233 m) during austral summer 2009–2010. Average (±1σ) mixing ratios at 1.0 m of NO and NO2, the latter measured for the first time on the East Antarctic Plateau, were 111 (±89) and 102 (±88) pptv, respectively. Atmospheric mixing ratios are on average comparable to those observed previously at South Pole, but in contrast show strong diurnal variability, with a minimum around local noon and a maximum in the early evening. The asymmetry in the diel cycle of NOx concentrations and likely any other chemical tracer with a photolytic surface source is driven by the diffusivity and height of the atmospheric boundary layer, with the former controlling the magnitude of the vertical flux and the latter the size of the volume snow emissions are diffusing into. In particular, the NOx emission flux estimated from concentration gradients was on average (±1σ) of 6.9 (±7.2) ×1012 molecule m−2 s−2 and is consistent with the 3-fold increase in mixing ratios in the early evening when the atmospheric boundary layer becomes very shallow. Dome C is likely not representative for the entire East Antarctic Plateau but illustrates the need of accurate descriptions for atmospheric boundary layer physics in atmospheric chemistry models. Calculated mean potential NO2 production rates from nitrate (NO3−) photolysis are only about 62% of the observed NOx flux and highlight uncertainties in the parameterization of the photolytic NOx snow source above Antarctica. A steady-state analysis of the NO2:NO ratios indicates high concentrations of peroxy radicals (HO2 + RO2) in the air above the snow and confirms the existence of a strongly oxidising canopy enveloping the East Antarctic Plateau in summer.
The concentrations of sulfur‐containing species (sulfate and methanesulfonate (MSA)) of aerosols collected at three coastal Antarctic sites (Neumayer, Dumont d'Urville, and Halley) have been studied in order to investigate the natural sulfur cycle at high southern latitudes. The multiple‐year data sets indicate annual mean concentrations of MSA and non‐sea‐salt (nss) sulfate of 38 and 151 ng m −3 at Neumayer (1983–1995) and 20 and 147 ng m −3 at Dumont d'Urville (1991–1995). On the basis of the study of a more limited time period (1991 and 1992), the Halley data set indicates significantly lower MSA and nss sulfate concentrations, 15 and 50 ng m −3 , respectiveley. The concentrations of both species exhibit a consistent and strong seasonal cycle with maxima from December to March and minima from May to September (from 2 to 4 and from 17 to 50 ng m −3 , for MSA and nss sulfate, respectively). These data, together with radionuclide studies ( 210 Pb and 10 Be), indicate that the marine biogenic source dominates the sulfur budget of the boundary layer of these regions throughout the year. The contribution of other sources, such as the long‐range transported sulfur from continents and to a lesser extent the stratospheric sulfate reservoir, remains weak when averaged over the year. Differences in the seasonal pattern of the two sulfur‐containing species, as well as intersite differences in the summer concentrations, are compared to several factors, mainly the spatial and temporal variations of the chlorophyll content of the surface ocean water and the seasonality of the sea ice cover. This permits investigation of the respective influence of marine regions emitting dimethylsulfide located either near the Antarctic continent (south of 60°S) or at more temperate latitudes. A strong correlation is found between the chlorophyll content of the Antarctic ocean and the level of total nss sulfur. Furthermore, nss sulfate and MSA deposition fluxes determined from firn and ice cores extracted at sites located at various distances from the ocean and various altitudes allow assessment of the spatial variation of the marine biogenic sulfur input in Antarctica.
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